Thank you Falkor and Steve.
I'm looking through a file at the Lewisham local studies centre which shows photocopies of the collapsed road. It was really quite dramatic and looks as though it knocked down the front garden wall of our next door neighbour as well as doing considerable damage in Mount Ash Road.
I'm told that part of the cause was big vehicles, including tankers delivering heating oil, driving down the lane. It really hadn't been built for that sort of thing!
The dustcart doesn't drive down there now, but we do still get big vehicles going down there - removal & delivery trucks, etc., plus Ocado regularly take chunks out of the trees. Everything shakes when they do so!